Period changes of δ Scuti stars and stellar evolution

Period changes of delta Scuti stars have been collected or redetermined from the available observations and are compared with values computed from evolutionary models with and without convective core overshooting. For the radial pulsators of Pop. I, the observations indicate (1/P) dP/dt values around 10(-7) year(-1) with equal distribution between period increases and decreases. The evolutionary models, on the other hand, predict that the vast majority should show increasing periods. This increase should be a factor of about ten times smaller than observed. For nonradial delta Scuti pulsators of Pop. I, the discrepancies are even larger. The behavior suggests that for these relatively unevolved stars the rate of evolution cannot be deduced from the period changes. The period changes of most Pop. II delta Scuti (SX Phe) stars are characterized by sudden jumps of the order of Delta P/P similar to 10(-6). However, at least one star, BL Cam, shows a large, continuous period increase. The variety of observed behavior also seems to exclude an evolutionary origin of the changes. Model calculations show that the evolutionary period changes of pre-MS delta Scuti stars are a factor of 10 to 100 larger than those of MS stars. Detailed studies of selected pre-MS delta Scuti stars are suggested.